1. Field of the Invention
The present invention relates generally to the field of holography. More particularly, the present invention relates to systems and methods for shearless digital hologram acquisition system suitable for use with “white light” (spectrally broadband) or laser illumination, or two-color illumination. For two-color (more than two colors is also possible) implementations, the two colors may either both be broadband (low or very low coherence) illumination or laser illumination.
In one implementation, an LED (broadband light source) or laser is used for illumination, a diffractive or holographic optical element is used to create the required phase shift in a reference arm, and the hologram is recorded on a digital camera. In one implementation, advanced alignment and signal processing systems and methods, combined with the shearless geometry, afford a one-dimensional (1-D) FFT (Fast Fourier Transform) so that the processing time is substantially diminished compared to prior art systems that require a two-dimensional (2-D) FFT.
2. Related Art
Prior methods of heterodyne (spatial carrier frequency) classical holography and of digital hologram acquisition have required both laser (coherent) illumination and that the reference and object (target) beams be combined at some angle (there is a shear between the two beams). Lasers have a number of problems, including high expense and generally requiring very extensive safety precautions, which makes them even more expensive. Additionally, since lasers have long coherence lengths (compared to broader band illumination sources), small reflections from optical surfaces will interfere with and make significant noise in the digital hologram. Previous methods have also required an angle (shear) between the two beams to create the spatially heterodyne fringe pattern that actually records the hologram. The shear is created by reflecting the reference beam from a mirror or beamsplitter so that it propagates at a different angle than the object (target) beam. For common path systems, such as a Michelson geometry, or the last leg of a Mach-Zender geometry to the digital recorder, this means that the beams separate spatially from one another, and in fact makes it impossible to use a Michelson geometry for systems with high magnification-the reference beam becomes so separated due to the shear that it is either clipped by the optics, does not overlay the object beam, or both. Even with the shorter common path Mach-Zender layout, shear between the two beams often causes problems in achieving adequate overlay of the beams. For low-coherence illumination source beams it is substantially impossible in either geometry to get an exact enough overlay to form fringes with the prior art sheared systems. Another problem with prior art digital hologram acquisition systems is that they require a two-dimensional (2-D) FFT (Fourier transform) and inverse FFT to separate the object wave phase and amplitude from the hologram. The 2-D FFT/inverse FFT requires large computational power or a long wait. Another considerable problem with prior art systems is that they have no method for measuring phase changes greater than one wavelength or two-pi radians in a shearless geometry. This is a substantial disadvantage for holographic metrology.
FIG. 1 shows a prior art digital holography system with a Michelson geometry, where the shear angle between the two beams is indicated as a. Note that for this particular case, nominally a high-magnification case, the reference and object beams no longer have any overlap, as indicated, and therefore cannot form a hologram.
There is therefore a particular need for systems and methods for 1) recording digital holograms in a shearless geometry, 2) recording digital holograms with broadband very short coherence length (both transverse and longitudinal) illumination, 3) recording digital holograms which extend the range of metrology substantially beyond one wavelength and 4) reducing the FFT computational requirements for separating the object wave phase and amplitude from the hologram.